Method of production of steel parts by quenching with temperature equalization at ms

ABSTRACT

This invention generally relates to a method for producing steel parts by creating multiphase microstructures within the steel parts. Typically, the microstructures comprise low-stress martensite with increased ductility and stabilized retained austenite. The microstructures may be formed by cooling the steel parts to a temperature around the martensite start temperature (Ms) and subsequently holding the steel part at a temperature around the Ms in order to equalize the temperature throughout the steel part. Afterwards, the formation of the microstructures and the desired properties is completed by subjecting the steel part to a final cooling in still air.

RELATED APPLICATIONS

This application claims the foreign priority benefit of Czech Patent Application Serial No. PV 2019-495 entitled “METHOD OF PRODUCTION OF STEEL PARTS BY QUENCHING WITH TEMPERATURE EQUALIZATION AT MS,” filed Jul. 30, 2019, the entire disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

The present invention generally relates to a method for producing steel parts, which leads to a microstructure containing low-stress martensite with improved ductility and stabilized retained austenite. Generally, the method involves heat treatment and/or thermomechanical treatment characterized by the steel parts being cooled from a temperature in the austenite region to a temperature around the martensite start temperature (“Ms”) and then held at the temperature around the Ms in a furnace and then finally cooled in air.

BACKGROUND ART

Steel parts are typically made from stock whose condition makes it easy to process. The structures of the steel parts usually require subsequent modification to attain the appropriate mechanical properties for the particular engineering application. The process which is often used to achieve this is a combination of quenching and tempering and is performed in multiple variants and with different parameters, depending on the material and the desired properties of the final structure. The typical structure after quenching is martensite, which exhibits high strength and hardness but very poor ductility owing to internal stress. This causes problems in parts under load, as they may suddenly fracture in service. Therefore, materials with martensitic structure are usually tempered after quenching. This produces a specific bainitic microstructure referred to as sorbite. Sorbite typically consists of fine carbides and bainitic ferrite. Although this treatment sequence reduces the strength of the material somewhat, it also increases its elongation, which is important to achieving the required operational safety of the part. Recently, treatment processes were developed which can lead to elongation levels around 10%, even in multiphase martensitic structures, but their use is complicated as it requires that quenching is interrupted at defined temperatures between the martensite start temperature (Ms) and the martensite finish temperature (“Mf”), typically around 200-300° C. Such processes require difficult quenching in molten salts, which entails subsequent cleaning of the surface and disposal of waste. Consequently, these processes are costly and are an environmental burden. Czech Patent No. CZ307645 describes a method of producing steel parts by creating a multiphase structure containing low-stress martensite with increased ductility and stabilized retained austenite by partial quenching in a quenching bath.

SUMMARY

One or more embodiments of the present invention generally concern a method for producing steel parts. Generally, the method comprises creating a multiphase structure, which comprises or consists of low-stress martensite with increased ductility and stabilized retained austenite, by partial quenching the steel part in a quenching bath. Consequently, the steel part is cooled from a temperature in the austenite region in the quenching bath so that the surface temperature of the steel part decreases to below the Ms by 5-50% to a value between the Ms and Mf. Meanwhile, the temperature of the interior of the steel part is above the Ms. Afterwards, the temperature throughout the steel part is equalized at the Ms temperature in equipment that maintains the Ms temperature. Subsequently, the steel part is removed from the furnace and cooled to ambient temperature.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention are described herein with reference to the following drawing figures, wherein:

FIG. 1 is a schematic representation of the treatment procedure and microstructural evolution in steel blanks with martensitic-austenitic microstructures.

DETAILED DESCRIPTION

This invention can find broad use in heat treatments and thermomechanical treatments of blanks from high-strength steels, namely in production of steel parts, typically for the machinery and transport industry.

The above-outlined shortcomings of today's production methods are eliminated by the present method, which is characterized by cooling a steel blank from a temperature in the austenite region by repeated cooling in boiling water. In one or more embodiments, the method of the present invention is carried out by gradually immersing the steel blank in a water bath and removing it from the bath to ensure that its surface temperature does not decrease considerably below the required cooling temperature, which is equal to the Ms temperature. Once the surface temperature of the blank is just below the Ms, the steel blank is removed from the bath and the surface temperature rises gradually due to to the heat flux from the interior of the steel blank to the surface of the steel blank. Alternatively, in one or more embodiments, cooling can be carried out in, for instance, a preheated oil bath instead of a water bath.

After removal from the bath and once the surface temperature increases, the steel blank can be repeatedly immersed in boiling water until heat is removed from the interior of the steel blank. As a result, the steel blank reaches a temperature just below the Ms, whereas the temperature in the core of the blank is above the Ms. Consequently, transformation of austenite initiates in the surface layers of the steel blank, and thereby partial transformation to martensite occurs. Since temperature does not decrease any closer to the Mf, only isolated martensite needles or plates form within supercooled austenite and divide austenite grains into segments. The amount of austenite may be between 5 to 25% by weight, depending on the cooling conditions in the sub-surface layer. However, in certain embodiments, martensitic transformation may be arrested because temperature does not decrease any more. Furthermore, carbon migrates from the super-saturated austenite into the surrounding austenite, which becomes stable due to super-saturation.

In one or more embodiments, the steel blank (may be interchangeably referred to as “part”) may be immersed in a bath (either water or oil) until the blank reaches a temperature 20° C. below the Ms. In certain embodiments, the bath may comprise a protective atmosphere to prevent oxidation and ignition. Furthermore, in one or more embodiments, the temperature of the quenching bath is maintained at a temperature that is below the Ms of the steel part by 5-50% of a value between the Ms and Mf. Upon reaching this temperature, the immersion process ends and the blank may be placed in a furnace kept at approximately 210° C., where it may be held for 20 to 180 minutes, depending on its size and shape. Afterwards, the blank may be removed from the furnace and cooled to ambient temperature.

In one or more embodiments, the steel is alloyed with Mn, Cr, and possibly other elements, but mainly with Si which may be present at 1.5 to 2.5% by weight. Therefore, the migrating carbon does not form carbides and remains free to migrate by diffusion. In the interior of the blank, where the temperature has not decreased to the Ms, metastable austenite remains, which cannot transform into martensite or bainite. Therefore, the interior of the blank continues to cool and the temperature of the interior eventually becomes closer to the surface temperature, which is kept around the Ms. In order to equalize temperature completely, the blank is placed in a heating box or in a furnace, which is at a temperature around the Ms.

Alternative methods may include, without limitation, immersion into a cooling medium whose temperature corresponds to a temperature that is required for temperature equalization and holding the part at this temperature. After holding, during which the temperatures in the part equalize, the part is removed and cooled in air. As a result, slow transformation to martensite occurs gradually throughout the part's volume. Since the temperature of the transformation is sufficiently high and the decrease in temperature is slow, favourable conditions are present for diffusion of carbon, which migrates into austenite and therefore stabilizes austenite in the vicinity of newly-formed martensite needles. The growth of adjacent martensite needles is therefore hindered by stabilized austenite and layers of retained austenite remain between the needles. Due to super-saturation, these layers are stable enough to survive the complete cooling to ambient temperature. As the super-saturated austenite in the regions between martensite cannot transform into ferrite and carbides, the resulting martensitic-austenitic structure retains high strength as well as toughness and exhibits an elongation of around 10%.

This invention can be further illustrated by the following examples of embodiments thereof, although it will be understood that these examples are included merely for the purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLES Example 1

A steel comprising the composition of TABLE 1 was used to produce a forged part. After forging, trimming, and sizing, a forged part from the steel was gripped in robot grippers. Based on a defined cooling sequence, the robot was programmed to immerse and remove the part from a cooling bath. In forged parts with irregular distribution of mass, the cooling intensity can be enhanced by partial immersion of the locations in which heat accumulation occurs. Once the part reached a temperature 20° C. below the Ms, the Ms temperature for this steel material being 190° C., the immersion sequence ended and the blank at this temperature was placed in a furnace kept at approximately 210° C., where it was held for 20 to 180 minutes, depending on its size and shape. It was then removed from the furnace and cooled to ambient temperature.

TABLE 1 Example of a suitable chemical composition of steel in weight percent C Si Mn Ni 0.42 2 2.45 0.45

EXAMPLE 2

Another example embodiment involves a procedure where a drawn part of thick-walled sheet of approximately 10 mm thickness, which was made of the steel composition of TABLE 1, whose temperature was approximately 930° C., was suspended from a holder of a cooling manipulator. The manipulator was programmed according to a pre-defined cooling sequence and was controlled to immerse the blank in and remove it from a cooling bath repeatedly. Once the blank surface reached a temperature 20° C. below the Ms, the Ms temperature for this material being 190° C., the immersion sequence ended and the blank at this temperature was placed into a furnace kept at approximately 210° C., where it is held for 20 to 90 minutes, depending on its size and shape. The blank was removed from the furnace and cooled to ambient temperature.

EXAMPLE 3

The third example embodiment involves a sequence in which an M28 screw at 930° C., which was made from the steel composition of TABLE 1, was immersed in a quenching oil bath at 200° C., which was under a protective atmosphere to prevent oxidation and ignition. The screw was removed from the cooling bath after 30 minutes and cooled to ambient temperature in still air.

Definitions

It should be understood that the following is not intended to be an exclusive list of defined terms. Other definitions may be provided in the foregoing description, such as, for example, when accompanying the use of a defined term in context.

As used herein, the terms “a,” “an,” and “the” mean one or more.

As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

As used herein, the terms “comprising,” “comprises,” and “comprise” are open-ended transition terms used to transition from a subject recited before the term to one or more elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up the subject.

As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

As used herein, the terms “including,” “include,” and “included” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

Numerical Ranges

The present description uses numerical ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds).

Claims Not Limited to Disclosed Embodiments

The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.

The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims. 

What is claimed is:
 1. A method for producing steel parts by creating a multiphase structure comprising low-stress martensite with increased ductility and stabilized retained austenite, wherein the method comprises: (a) at least partially quenching a steel part in a quenching bath to thereby cool the surface of the steel part from a temperature in the austenite region to a temperature below the martensite start temperature (“Ms”), wherein the temperature of the surface of the steel part decreases by 5 to 50% of the interval between the Ms and martensite finish temperature (“Mf”), wherein the temperature of the interior of the steel part is maintained above the Ms, (b) equalizing temperature throughout the steel part to the Ms in equipment that maintains the Ms; (c) removing the steel part from the equipment; and (d) cooling the steel part to ambient temperature to thereby form a finished steel part comprising said multiphase structure.
 2. The method according to claim 1 wherein, during said quenching, the steel part is repeatedly immersed in and removed from the quenching bath.
 3. The method according to claim 1 wherein the temperature of the quenching bath is maintained at the Ms +/−50% of the interval between the Ms and Mf below the Ms for the steel part.
 4. The method according to claim 1 wherein the quenching bath comprises a protective atmosphere. 